Slamming terrain markers

'ey there, I'm reading the comprehensive rules and can't figure out how this works in 4E:

Can you Slam terrain markers like Titania's Underbrush or Sonnia's Inferno? Or any of the other ones, really?

Slam says it can destroy non-strategy markers, which these markers are. But it can only destroy destructible terrain, which these markers are not...

These markers are considered terrain because they have at least one terrain trait. And none have the destructible trait, making them essentially indestructible terrain.

So..... Are they destructible markers or indestructible terrain? Can you remove them because they're markers? Or you can't because they "count as terrain"?